The present invention relates to a method for fabrication of a semiconductor device which comprise a polishing process for planarizing the surface pattern in fabrication of an integrated semiconductor circuit and an apparatus suitable for processing the semiconductor device.
A fabrication process for fabricating a semiconductor device comprises many processing steps. A metallization process is described with reference to FIG. 2(a) to FIG. 2(f) as an example of a process comprising a polishing step.
FIG. 2(a) shows a cross section of a wafer on which the first wiring layer has been formed. A dielectric film 2 is formed on the surface of a wafer substrate 1 having a transistor unit (not shown in the drawing), and a wiring layer 3 of aluminum is formed thereon. Because a hole is provided on the dielectric film 2 for serving to connect to the transistor, the surface of a portion 3xe2x80x2 is somewhat concave above the hole. In the step for forming the second wiring layer shown in FIG. 2(b), a dielectric film 4 and aluminum layer 5 are formed on the first layer, and further a photo resist layer 6 is formed to pattern the aluminum layer 5 in the form of wiring. Next as shown in FIG. 2(c), the circuit pattern is transferred by exposing on the photo resist 6 by use of a stepper 7. At that time, the focus can not be adjusted both on the convex surface and the concave surface of the photo resist layer 6, and a serious defocus problem is caused.
To solve the problem, planarization processing of the substrate surface is carried as described herein under. Subsequently to the processing step shown in FIG. 2(a), a dielectric film is formed as shown in FIG. 2(d) and then the dielectric film is polished to the predetermined level 9 in the drawing to planarize the surface by a method described hereinafter, and the surface of the dielectric film 4 is planarized as shown in FIG. 2(e). Thereafter, an aluminum layer 5 and photo resist layer 6 are formed and exposed by use of a stepper 7 as shown in FIG. 2(f). In this case, the defocus problem is not caused because the surface of the photo resist layer 6 is planer.
CMP (Chemical Mechanical Polishing) process which has been used generally to planarize a dielectric pattern is shown in FIG. 3. A polishing pad 11 which is adhered on a platen 12 is being rotated. For example, a foamed polyurethane resin sheet which is formed by slicing a foamed polyurethane resin block is used as the polishing pad. However, generally the material of the polishing pad is selected in view of property and surface structure of materials depending on type of the object to be processed and desired surface finish roughness. On the other hand, a wafer substrate 1 to be processed is fixed on a wafer holder 14 with interposition of an elastic backing pad 13. A wafer substrate 1 is pressed on the surface of the polishing pad 11 while the wafer holder 14 is being rotated, polishing slurry 15 is supplied on the polishing pad 11 to remove and planarize the convex portion of the dielectric film 4 on the wafer surface.
For polishing a dielectric film of such as silicon dioxide, generally fumed silica is used as the polishing slurry. The fumed silica is a suspension formed by suspending fine silica particulate having a diameter of about 30 nm in an alkaline solution containing alkali such as ammonia or potassium hydroxide. A plane surface is obtained without damage by use of fumed silica.
In CMP processing using abrasive grain suspension, an object is polished while polishing slurry is fed between a polishing pad and the object, the following problem arises due to use of the polishing pad and polishing slurry.
First, the capability of planarization is not sufficient because the Young""s modulus of the polishing pad is not high. Because the polishing pad is in contact not only with convex portion but also with concave portion of the wafer surface because of the pressure during polishing. That is particularly true for larger pattern. The planarizable maximum pattern size is several mm width for a method in which the polishing pad is used, and it is difficult to sufficiently planarize a pattern having a size as large as several cm which is required for, for example, DRAM. Next, the special caution is needed when dealing with the polishing slurry, the special caution results in high cost. Dried polishing slurry can not be removed easily, and residual polishing slurry is the source of dust which adversely affects the cleanliness in a clean room. Abrasive grains in the polishing slurry aggregate each other with time to form aggregated particles. The aggregated particles cause damage such as scratch. The polishing slurry generally contains alkali, and the apparatus should be adapted to alkali. As the result, a polishing slurry supplying equipment to be used exclusively is required and the polishing slurry is expensive itself. Therefore, the processing cost for a CMP processing method in which abrasive grain suspension is used is high. Further, there arises a problem that the shape of the surface of a polishing pad is deformed with using and the removal rate (efficiency of polishing) decreases. To resume the removal rate, a polishing pad is reclaimed every time when one wafer substrate is processed or when processing simultaneously, which reclamation is generally called as dressing. A file referred to as dresser which is formed by electrically depositing diamond abrasive grains is used to roughen the surface of the polishing pad, and the removal rate is resumed.
As the wafer substrate planarization processing technique for solving the problem associated with CMP processing by use of abrasive grain suspension, a part of the inventors of the present invention proposed the planarization technique with grindstone in which fixed abrasive was used (International application open laid; WO 97/10613).
FIG. 4 is a schematic diagram for describing the planarization processing using grindstone. The basic structure of the apparatus is the same as that used in CMP polishing technique in which a polishing pad and abrasive grain suspension are used, but this apparatus is different from the conventional CMP polishing technique in that a grindstone 16 containing abrasive grains of cerium oxide instead of a polishing pad. It is possible to polish by merely supplying deionized water which contains no abrasive grain instead of fumed silica slurry as a polishing supply. This method in which a grindstone is used as polishing tool is excellent in capability of planarizing pattern topography, and it is possible to sufficiently planarize a pattern having several mm width, which is difficult to be planarized by the conventional method. The process cost is reduced by employing this method because a grindstone which is excellent in utilization of abrasive grain, is used instead of polishing slurry which is inferior in utilization.
Japanese Unexamined Patent Publication No. Hei 7-249601 discloses a polishing technique in which a grindstone for polishing bare wafers is cleaned by jetting high pressure fluid or by use of a brush, however this conventional technique addresses neither on the method for polishing a wafer on which a device is formed nor the method for planarization of a wafer on which a device is formed.
On the other hand, U.S. Pat. No. 5,624,303 discloses a method in which a polishing pad containing abrasive grains to which treatment for preventing breaking down of abrasive grain is applied, and U.S. Pat. No. 5,782,675 discloses a method for conditioning to prevent breaking down of abrasive grains of a polishing pad containing abrasive grains.
The techniques in which a grindstone is used for polishing is excellent in low cost and planarization capability, however involved in the problem as described herein under.
First, the removal rate of the method in which only deionized water is used as the process fluid is as low as about ⅓ of the removal rate of a method in which abrasive grain suspension is used. In the polishing in which a grindstone is used, the removal rate falls down with time of polishing similarly to the polishing method in which a polishing pad and polishing slurry are used. It is difficult to adjust the polishing thickness to a desired value unless the removal rate is maintained at a constant value.
The mechanism of deterioration of removal rate using a grindstone is not necessarily the same as that of removal rate using a polishing pad and polishing slurry. In the case of combination of a polishing pad and polishing slurry, abrasive grains are not fixed on a polishing pad, which is a polishing process tool, and free from the polishing pad during polishing, on the other hand in the case of a grindstone, abrasive grains are fixed on a polishing process tool itself and the fixed abrasive grains are involved in polishing, there is big difference in mechanism between the former and the latter. The removal rate deterioration in polishing using a polishing pad and a liquid (slurry) containing abrasive grains is attributed to the decrement in abrasive grain retainability due to deformation of the polishing pad surface and to the increment in effective contact surface. On the other hand, the removal rate deterioration in polishing using a grindstone is attributed to the decrement in the number of abrasive grains exposed on the grindstone surface and the deterioration of chemical activity of the abrasive grain surface. To activate the surface of a grindstone so that the removal rate does not fall down, a method based on a principle which is different from a principle for the other polishing method is required.
It is an object of the present invention to provide a method for fabricating a semiconductor device including a polishing step for polishing at high removal rate so as to control the polishing thickness desiredly.
It is another object of the present invention to provide a processing apparatus for polishing at high removal rate so as to control the polishing thickness.
To achieve this and another objects, a method for fabricating a semiconductor device of the present invention includes grindstone surface activation treatment carried out when a concave/convex pattern is planarized by polishing a semiconductor wafer having concave/convex pattern thereon by use of a grindstone comprising abrasive grains and material for holding the abrasive grains onto which the semiconductor wafer is pressed with relative motion.
The grindstone surface activation treatment maybe carried out by use of a brush pressed onto the grindstone or by transmitting ultrasonic wave or acoustic wave having a frequency of 10 kHz or higher. The surface activation treatment is by no means limited to the methods, otherwise the surface activation treatment may be carried out by pressing a diamond dresser onto the grindstone.
A single substance or mixture containing two or more substance of silicon dioxide, cerium oxide, aluminum oxide, silicon carbide, manganese oxide, and zirconia is preferably used as the abrasive grain, and an organic resin is preferably used as the material for holding the abrasive grain. A grindstone disclosed in the PCT application, PCT/JP 95/01814 (International Laid Open No. WO 97/10613), may be used as the grindstone to be used in this invention. It is preferable that a grindstone contains micro-pores and the micro-pores having a diameter of 1 xcexcm or smaller occupy at least 95% (2"sgr") of the total pore volume. A liquid which is deionized water or deionized water containing additives is supplied onto the surface of a grindstone as a processing liquid.
To achieve this and another objects, a processing apparatus for processing a semiconductor device is provided with the first means for holding a semiconductor wafer having the concave/convex pattern formed on the surface, a grindstone comprising abrasive grains and material for holding these abrasive grains, the second means for pressing the semiconductor wafer surface onto the grindstone and for moving the semiconductor wafer surface relatively to the grindstone, and the third means served for surface activation treatment.
A brush, or a means for generating ultrasonic wave or acoustic wave having a frequency of 10 kHz or higher and means for transmitting the ultrasonic wave or acoustic wave to the grindstone may be used as the third means. The grindstone is used as the grindstone.
In the grindstone surface activation treatment, a processing liquid which is deionized water or deionized water containing additives is supplied to the surface of a grindstone. A dispersant or pH buffer is used as the additives. It is preferable that the supply flow rate of processing liquid is 0.14 ml/cm2 or less per minute per unit area of a grindstone. Abrasive grains and resin bonded weakly are liberated in a large amount from the surface of a grindstone by surface activation treatment. The increment of liberated abrasive grain concentration contributes to the increment of removal rate. It is preferable that the supply flow rate of processing liquid onto the surface of a grindstone is not excessive to maintain the liberated abrasive grain concentration high. FIG. 21 shows the relation between the amount of processing liquid and removal rate. The amount of processing liquid must be controlled at the optimal point to obtain high removal rate, the removal rate decreases if the amount of processing liquid is excessive.
In the case in which a brush is used as the surface activation treatment means, it is preferable that a brush is pressed further a certain distance toward the grindstone side from the position where the end of bristles of the brush just touches on the surface of a grindstone, and the pressing distance is preferably in a range from 0.1 to 5 mm. The contact of a brush is unstable and the removal rate is low if the pressing distance is shorter than 0.1 mm and on the other hand a grindstone can be damaged if the pressing distance is longer than 5 mm.
The role of a brush used in the present invention is to brush out process dust and broken down abrasive grains and to expose fresh abrasive grain surface. In the method for conditioning a polishing pad having fixed abrasive grains with a brush disclosed in the U.S. Pat. No. 5,782,675, the brush is used for soft conditioning in which fixed abrasive grains are not broken down, therefore the U.S. Pat. No. 5,782,675 is different from the present invention in principle.
Treatment which is so-called truing is applied periodically to correct the surface configuration of a grindstone and to maintain the surface planar. It is preferable that the planarity of the grindstone surface is 10 xcexcm or lower. For truing, controlled depth machining may be applied. In this method for controlled depth machining, a ring or disk having a diameter of 30 to 70 mm on which abrasive grains of hard material such as diamond is embedded is rotated at a rotation speed of 3000 to 10000 rpm and the tool is moved relatively in the grindstone surface with maintaining the distance between the tool and grindstone in a constant value, thus the grindstone surface is trued at a high precision. In such controlled depth processing, the high positioning accuracy of the height of a tool results in the high planarity in principle. It is preferable in the present invention that the positioning accuracy of the height of a tool is 1 xcexcm or lower. A correction ring or dresser which has been used heretofore to correct the tool plane in polishing processing such as lapping or CMP can not result high planarity because a correction ring or dresser cuts the tool surface with a constant pressure (constant pressure processing). Because a method in which a fixed abrasive grain polishing pad and a brush are used disclosed in the U.S. Pat. No. 5,782,675 is classified to the constant pressure processing in which the pressure of a brush is set, therefore the method can not likely result in high planarity.
By applying the controlled depth truing processing, the processing defect of wafer such as scratch is decreased and the uniformity within wafer of amount of removal is increased. Because the removal thickness of a grindstone by the truing processing is as small as several xcexcm from the surface of the grindstone, the service life of a grindstone is very long.
The grindstone surface activation treatment using an abrasive grain supply source other than liquid may be applied as the grindstone surface treatment. A grindstone formed by bonding abrasive grains with resin, iced material formed by icing a liquid containing abrasive grains, or gel or aerosol of a liquid containing abrasive grains may be used as the abrasive grain supply source.
The first and second objects are achieved by applying surface treatment using a polishing pad and an abrasive grain supply source other than liquid instead of the grindstone for polishing a semiconductor wafer. At that time, a grindstone formed by bonding abrasive grains with resin, iced material formed by icing a liquid containing abrasive grains, or gel or aerosol of a liquid containing abrasive grains may be used as the abrasive grain supply source.